[1.] Field of the Invention
The present invention relates to a thin flat cable configured to transmit radio frequency signals.
[2.] Description of Related Art
Conventionally, a coaxial cable is a typical example of a radio frequency line for transmitting radio frequency signals. A coaxial cable includes a central conductor (signal conductor) shaped to extend in one direction (shaped to extend in the direction of signal transmission), and a shield conductor provided concentrically along the outer peripheral surface of the signal conductor.
Incidentally, as radio frequency devices including mobile communications terminals have become increasingly smaller and thinner in recent years, it is not possible to secure a space for disposing a coaxial cable inside the terminal housing in some cases.
Accordingly, attention is being given to use of a flat cable as discussed in each of International Publication No. WO 2011/007660 and Japanese Registered Utility Model No. 3173143 for such a terminal housing. Although a flat cable has a larger width than a coaxial cable, a flat cable can be reduced in thickness, which proves particularly advantageous for cases such as when there is only a small gap inside the terminal housing.
The flat cable discussed in each of International Publication No. WO 2011/007660 and Japanese Registered Utility Model No. 3173143 has a triplate strip line structure as its basic structure.
FIG. 13 is an exploded perspective view illustrating a structure of the flat cable discussed in International Publication No. WO 2011/007660. In FIG. 13, the base material of the flat cable is omitted, and only the structure of conductors is depicted.
A conventional flat cable 10P includes a first ground conductor 20, a second ground conductor 30, a signal conductor 40, and a base material (not illustrated). The first ground conductor 20, the second ground conductor 30, and the signal conductor 40 are flat film conductors having an elongated shape extending in a first direction that is the direction of signal transmission.
The first ground conductor 20, the second ground conductor 30, and the signal conductor 40 are disposed at a predetermined distance in a direction orthogonal to their respective flat film surfaces, so that these flat plate surfaces become parallel to one another.
The signal conductor 40 is disposed between the first ground conductor 20 and the second ground conductor 30.
The second ground conductor 30 includes an elongated conductor 31 and an elongated conductor 32 which extend in the first direction. The elongated conductor 31 and the elongated conductor 32 are spaced apart along a second direction that is orthogonal to the first direction. As viewed in a direction orthogonal to their flat film surfaces, the elongated conductor 31 and the elongated conductor 32 are disposed so as to sandwich the signal conductor 40, without overlapping the signal conductor 40.
The elongated conductor 31 and the elongated conductor 32 are partially connected by a plurality of bridge conductors that are shaped to extend along the second direction. The plurality of bridge conductors 33 are disposed at predetermined spacings along the first direction. As a result, the second conductor 30 has a ladder-like configuration with a plurality of openings 341, 342, and 343 provided along the first direction.
The second ground conductor 30 and the first ground conductor 20 are connected by a plurality of via-hole conductors 511P, 512P, 521P, and 522P.
The plurality of via-hole conductors 511P, 512P, 521P, and 522P are formed in areas of the elongated conductor 31 and elongated conductor 32 which connect with each of the bridge conductors 33. More precisely, each of the plurality of via-hole conductors 511P, 512P, 521P, and 522P is disposed at the center position in the width direction (direction parallel to the first direction) of the corresponding bridge conductor 33.
However, the flat cable disclosed in each of International Publication No. WO 2011/007660 and Japanese Registered Utility Model No. 3173143 which has the structure illustrated in FIG. 13 has problems described below. FIG. 14 is an exploded perspective view for explaining problems of the conventional flat cable 10P.
In the conventional flat cable 10P, it is the portion of the bridge conductors 33 (331 and 332) whose flat film surface is opposed to the second ground conductor 30 and the signal conductor 40.
Therefore, when transmission of a radio frequency signal causes a current to flow through the signal conductor 40, countercurrents 911, 912, 921, and 922 are generated with the bridge conductors 331 and 332 as countercurrent generation points 901 and 902, respectively.
The countercurrent 911 flows from the bridge conductor 331 to the elongated conductor 31, with the countercurrent generation point 901 of the bridge conductor 331 as a starting point.
The countercurrent 911 flows to the first ground conductor 20 via the via-hole conductor 511P located in close proximity to the bridge conductor 331. The countercurrent 911 also flows to the first ground conductor 20 via an area 311 of the elongated conductor 31 which is located between the bridge conductors 331 and 332, and the via-hole conductor 521P located in close proximity to the bridge conductor 332.
The countercurrent 921 flows from the bridge conductor 332 to the elongated conductor 31, with the countercurrent generation point 902 of the bridge conductor 332 as a starting point.
The countercurrent 921 flows to the first ground conductor 20 via the via-hole conductor 512P located in close proximity to the bridge conductor 332. The countercurrent 921 also flows to the first ground conductor 20 via an area 312 of the elongated conductor 31 which is located between the bridge conductors 332 and another bridge conductor, and a via-hole conductor located in close proximity to the other bridge conductor.
In this way, in the conventional flat cable 10P, the countercurrents 911 and 921 generated from different countercurrent generation points 901 and 902, respectively, concentrate on the via-hole conductor 521P. Although not described in detail, such current concentration also occurs for the other via-hole conductor provided to the elongated conductor 31.
The countercurrent 912 flows from the bridge conductor 331 to the elongated conductor 32, with the countercurrent generation point 901 of the bridge conductor 331 as a starting point.
The countercurrent 912 flows to the first ground conductor 20 via the via-hole conductor 512P located in close proximity to the bridge conductor 331. The countercurrent 912 also flows to the first ground conductor 20 via an area 321 of the elongated conductor 32 which is located between the bridge conductors 331 and 332, and the via-hole conductor 522P located in close proximity to the bridge conductor 332.
The countercurrent 922 flows from the bridge conductor 332 to the elongated conductor 32, with the countercurrent generation point 902 of the bridge conductor 332 as a starting point.
The countercurrent 922 flows to the first ground conductor 20 via the via-hole conductor 522P located in close proximity to the bridge conductor 332. The countercurrent 922 also flows to the first ground conductor 20 via an area 322 of the elongated conductor 32 which is located between the bridge conductors 332 and another bridge conductor, and a via-hole conductor located in close proximity to the other bridge conductor.
In this way, in the conventional flat cable 10P, the countercurrents 912 and 922 generated from different countercurrent generation points 901 and 902, respectively, concentrate on the via-hole conductor 522P. Although not described in detail, such current concentration occurs likewise for the other via-hole conductor provided to the elongated conductor 32.
As described above, in the conventional flat cable 10P, countercurrents generated in the second ground conductor 30 concentrate on a via-hole conductor. This makes current density higher in the neighborhood of the via-hole conductor, causing a localized increase in resistance in the vicinity of the via-hole conductor. Consequently, a potential difference develops between the second ground conductor 30 and the first ground conductor 20, causing unwanted resonance. Therefore, the transmission characteristics of the flat cable 10P as a radio frequency line deteriorate. In particular, as compared with the bridge conductor 332, via-hole conductors have high resistivity and do not readily allow current to flow through. If currents flowing through bridge conductors concentrate on a via-hole conductor, the resistance encountered when countercurrents flow becomes high, leading to an increase in the transmission loss of the transmission line as a whole.